Noninvasive or nondestructive inspection of objects may be accomplished by irradiating the objects with X-rays and collecting an X-ray image by means of a suitable detector. To bring out specific characteristics or structures of an object, the dual energy method is used, with which the object is irradiated with X-rays at two different energy spectra (or frequency or wavelength spectra) and one image is registered for each of the two spectra. This utilizes the effect that certain materials or material structures exhibit different absorption or attenuation behaviors for radiation at different wavelengths or frequencies. The two images can then be combined by computer to improve the ability to detect certain structures. For example, a difference image can be generated by combining the image data, which are preferably in digital form, pixel by pixel, by a weighted subtraction. The output image can then be tested either visually or by means of a pattern recognition method to see whether certain criteria are satisfied. For example, this allows foreign objects in liquid or paste-like products to be detected more reliably than would be possible in the case of radiation with a single narrow band or with a broadband X-ray (for example, DE 690 11 194 T2).
However, this dual energy process, which can also be designed as a multi-energy process if more than two X-rays that differ in wavelength or frequency or energy spectrum are used, results in a higher cost than irradiation with one X-ray that has only a single, constant spectrum.
The dual energy method or multi-energy method is known in a number of variations. For example, two or more radiation sources that have different spectral characteristics can be used. They can be combined in each case with one detector, or a corresponding number of detectors. For example, U.S. Pat. No. 6,370,223 B1 shows a device with two radiation sources and two detectors, where each pair consisting of a radiation source and associated detector is situated at different positions on a conveyor belt for transporting objects that are to be inspected. However, this also results in a double expense for hardware to generate the two images for irradiation with different energy spectra.
Also known is the use of one or more filters (especially edge filters) in order to generate, from a broadband source, a beam with only a portion of the original spectrum and the use of the same detector for each image acquisition (for example, U.S. Pat. No. 3,665,184).
It is also possible to control one and the same X-ray source differently at successive points in time so that not only is the radiation power (constant over the entire spectrum) altered, but a change of the spectral characteristic is also achieved (JP 2010172590A).
However, these methods are hardly applicable for rapidly moving objects when only one detector is used, since it is not possible to generate images for identical parts of the object or indeed the entire object practically simultaneously at justifiable expense. For this, one would have to use extremely fast detector devices in order to register the at least two images for the different radiation spectra as closely as possible to each other in succession. In addition, the switching from the one radiation source to the other would also have to take place correspondingly rapidly. The solution of using a flat panel detector and bringing images acquired at short time intervals, which thus are spatially shifted on the detector, into alignment by computer means, would also result in a corresponding expense. Such flat panel detectors, which are mostly made as digital detectors with pixels arranged in a matrix pattern, are relatively expensive and if the resolution is high and exposure time is short, also lead to only low radiation energy being detected per pixel. Noise problems can arise because of this.
A dual energy method for investigation of the human body is known from CN 101455573 A, in which a spatial filter is placed immediately before a flat panel sensor. The spatial filter has strips of two different types, which are alternately arranged. The different strip types have materials that have different densities in the irradiation direction. The different filter regions act so that, in one of the strip-shaped regions, the radiation is not dampened or is only weakly dampened, and in the other of the strip-shaped regions, a (stronger) filter effect (high pass, low pass or band pass filter effect) exists, or a different edge filter effect exists in each of the two regions. This allows two images of the same object that contain different information to be generated simultaneously, with a single exposure. These images can then be combined by computer. Here, the complete image information (in the required resolution) can also be calculated in the total cross section (Nyquist-Shannon sampling theorem) with a single arrangement of strips, if the local sampling interval generated across the lengthwise direction of the strips is small enough that a structure to be imaged, which has a specified minimum size (in the transverse direction), is sampled at least twice. This can take place, for example, through interpolation processes or by means of a fast Fourier transform. The calculated image information can then be combined by a sign-dependent addition, and a weighted subtraction of the two images can, of course, also be realized. Through this, the advantage of the dual energy method can be combined with the advantage of using a single detector and a single radiation source (a broadband source or one with at least two narrow-band energy regions).
The method described in CN 101455573 A and the corresponding device are not well suited for use in an industrial process, for example for quality control of products, where it is often necessary to detect the internal and/or external structure of rapidly moving products. In particular, the use of a sufficiently large flat panel detector with sufficient resolution in industrial use would be much too costly.